Conventionally, in the art of fabrication of semiconductor devices, for example, a processing device is frequently used in order to form the fine circuit structure of a semiconductor device. This processing device generates plasma from a processing gas, so as to perform a process on a desired portion of the semiconductor device by the action of the plasma.
In recent years, a processing device has been developed that includes a detection mechanism for optically detecting the processed state of a substrate to be processed, for example, the thickness of a layer to be etched, or the like, and performs a process on the substrate while detecting the layer thickness or the like, thereby performing the process to a desired result.
FIG. 6 schematically shows the structure of this processing device. As shown in FIG. 6, this processing device includes a cylindrical vacuum chamber that is formed of aluminum, for example, in such a manner that the chamber can be closed airtight. The vacuum chamber forms a processing room.
In the vacuum chamber 1, a placing table (suscepter) 2 is provided for supporting approximately horizontally a semiconductor wafer W as a substrate to be processed in such a manner that the surface to be processed faces upward. The placing table 2 also serves as a lower electrode and is connected to a radio frequency power supply 4 via a matching box 3. This placing table 2 is typically provided with an electrostatic chuck (not shown) for absorbing and holding the semiconductor wafer W, a temperature control system (not shown) for controlling the temperature of the semiconductor wafer W to be at a predetermined temperature, and the like.
On the other hand, a shower head 6 is provided on the ceiling of the vacuum chamber 1 above the placing table 2. The head 6 includes a plurality of gas delivery holes 5 that face the placing surface of the placing table 2 on which the semiconductor wafer W is to be placed. On the back of the shower head 6, a gas diffusion gap 7 is provided.
The ceiling of the vacuum chamber 1 is provided with a gas introducing port 9, to which a gas feed pipe 8 is connected. Under the gas introducing port 9 a circular groove 9a, having a larger diameter than that of the gas introducing port 9, is provided to be in communication with the gas introducing port 9. A baffle plate 11, in the form of a circular disk having a plurality of gas feed holes 10 provided therein, as shown in FIG. 7, is arranged to close the lower opening of the circular groove 9a. 
Thus, a predetermined processing gas supplied from the gas feed pipe 8 is introduced into the gas diffusion gap 7 on the back of the shower head 6. The processing gas is introduced from the gas introducing port 9 through the gas feed holes 10 of the baffle plate 11. The processing gas thus introduced is diffused within the gas diffusion gap 7, and is then delivered as uniformly as possible from the respective gas delivery holes 5 toward the semiconductor wafer W.
Moreover, light transmitting windows 12 and 13 formed by light transmitting members are provided at the center of the shower head 6, and in a corresponding region of the ceiling of the vacuum chamber 1, respectively. Through the light transmitting windows 12 and 13, the processed state (layer thickness) of the semiconductor wafer W can be detected by a detection mechanism provided outside the vacuum chamber 1, for example, a layer-thickness detection mechanism 14. The layer-thickness detection mechanism 14 is provided with a driving mechanism 14a for moving the layer-thickness detection mechanism 14 so as to be able to measure the layer thickness in a desired portion of a semiconductor chip.
At the bottom of the vacuum chamber 1 is provided an exhaust port 15, connected to a vacuum exhaust mechanism, which is not shown. Thus, the atmosphere in the vacuum chamber 1 can be set to a predetermined vacuum atmosphere. In addition, in the outside of the sidewall of the vacuum chamber 1, an annular magnetic field generation mechanism 16 is provided. This magnetic field generation mechanism 16 can generate a predetermined magnetic field within the vacuum chamber 1. The magnetic field generation mechanism 16 is provided with a not-shown driving mechanism that can rotate the magnetic field generation mechanism 16 around the vacuum chamber 1.
Thus, a processing device having the aforementioned structure is arranged to uniformly supply a processing gas toward the semiconductor wafer W by distributing the processing gas introduced from the gas introducing port 9 by the components such as the baffle plate 11, the gas diffusion gap 7, and the shower head 6.
However, as described above, in the processing device provided with the layer-thickness detection mechanism 14 for detecting a thickness of a layer on the semiconductor wafer W, or the like, it is necessary to detect the thickness of the layer at the center of the semiconductor wafer W. Thus, the light transmitting windows 12 and 13 for the layer-thickness detection mechanism 14 have to be arranged at the center of the vacuum chamber 1. Consequently the gas introducing port 9 has to be arranged away from the center of the vacuum chamber 1. This makes the process rate of a semiconductor wafer W faster in the region directly below the gas introducing port 9 than in other regions, thus degrading in-plane uniformity of the process.
Moreover, since the light transmitting windows 12 and 13 are provided above the central portion of the semiconductor wafer W, the processing gas cannot be supplied toward the semiconductor wafer W in this region. Thus, the process rate in the central portion of the semiconductor wafer W tends to become lower than that in other regions. This further degrades the in-plane uniformity of the process.